As is known, a motor vehicle drivetrain comprises a starting and gear-shift clutch between a drive engine and a transmission. Particularly in the case of drivetrains with automatically shifting transmissions, the starting and gear-shift clutch is usually also made to be actuated automatically, so that a driver of the vehicle is largely relieved of the need to actuate the gear shifts and the clutch. To actuate such a clutch, an actuator that can be controlled by a control unit is used, which in most applications is actuated by a pressure medium but can also be made to be actuated electrically.
Pressure-medium-actuated clutch actuators consist, in the simplest case, of a cylinder in which a piston is fitted and able to move axially. The piston is connected by a piston rod to a clutch element which, when moved axially, keeps the friction linings of the clutch separated against the restoring force of springs or brings them together, thereby engaging or disengaging the clutch. In relation to this axial movement, the clutch actuator is not made self-locking, so if there is a pressure drop its piston allows the clutch to move back to its starting position, thereby engaging it.
To actuate pressure-medium-actuated clutch actuators, electromagnetic switching valves are usually used, which can be controlled by a control unit on the basis of information from sensors and control programs. Depending on their switch position these switching valves provide, for at least one pressure space of the clutch actuation cylinder, a pressure medium connection to a pressure delivery line, the seal the cylinder space relative to the outside, or enable it to be drained into a pressure medium tank. By virtue of these control functions the piston of the clutch actuation cylinder is axially displaced or maintained in a particular position, so that the starting and gear-shift clutch can be disengaged, engaged or operated in slipping mode.
As is known from DE 101 61 742 A1, certain hydraulic, electro-hydraulic or electric components of a motor vehicle even such as switching valves and adjusting motors are, for reasons of cost among others, not designed to last for very long times, so that to avoid damaging such components a maximum permissible duration of operation within a reference time period should not be exceeded.
Now, if a switching valve for a clutch actuation cylinder or a pressure medium line that connects this switching valve to the clutch actuation cylinder has a leak, in order to open the clutch, and depending on the size of the leak, the switching valve must be actuated until the control unit has determined, on the basis of sensor information, that the clutch has disengaged to the desired extent, whereupon it transmits an engaging command or at any rate no further disengaging commands to the switching valve in question. If the leak is comparatively large this can result with the switching valve remaining actuated for a very long time in order to keep the clutch disengaged. Such a long an actuation time can be damaging for the life of the switching valve and may even lead to its sudden failure. In addition, situations must be avoided in which, for example, when a gear is engaged and the starting and gear-shift clutch is disengaged, a leak results in the undesired driving circumstance that the clutch engages inadvertently because of the leak.
This operating behavior of the starting and gear-shift clutch can also occur with clutch actuators which are not actuated by pressure medium and therefore have no switching valves. For example, in the case of a clutch actuator based on a spindle drive powered by an electric motor and not designed to be self-locking, it must be ensured that if a component fails or if the electric voltage supply is briefly interrupted, for example because of a loose contact, no driving situation arises in which the starting and gear-shift clutch engages inadvertently while a gear is engaged.